1. Field of the Invention
This invention relates to a ball spline, and more particularly to a ball spline suited for use in a ball spline-equipped ball screw.
2. Related Art
There is known one conventional ball spline-equipped ball screw comprising a screw shaft having a screw groove and a plurality of axially-extending linear spline grooves formed in an outer peripheral surface thereof, a screw-purpose nut having the screw shaft passed therethrough and including a screw-purpose ball circulation passage corresponding to the screw groove of the screw shaft, a spline-purpose outer tube having the screw shaft passed therethrough and including spline-purpose ball circulation passages corresponding to the spline grooves of the screw shaft, a plurality of screw-purpose balls provided in the screw-purpose ball circulation passage of the screw-purpose nut, and a plurality of spline-purpose balls provided in the spline-purpose ball circulation passages of the spline-purpose outer tube (see JP-A-6-300106).
The ball spline portion serves to bear a torque acting in a rotational direction, and in the conventional ball spline, for example, three of six rows of main passages bear a torque acting in a clockwise direction, whereas the other three rows of main passages bear a torque acting in a counterclockwise direction.
Namely, as shown in FIG. 5, a conventional ball spline 41 comprises a screw shaft 42 having a plurality of axially-extending linear spline grooves 43 formed in an outer peripheral surface thereof, a spline-purpose outer tube 44 having the screw shaft 42 passed therethrough and including spline-purpose ball circulation passages corresponding to the spline grooves 43 of the screw shaft 42, a plurality of spline-purpose balls 45 provided in the spline-purpose ball circulation passages of the spline-purpose outer tube 44, and a cage 53. The spline-purpose ball circulation passages are formed by respective main passages 46A and 46B (in which the spline-purpose balls 45 roll) and respective return passages 47 each communicating with opposite end portions of the corresponding main passage 46A, 46B.
The main passages 46A and 46B of the spline-purpose ball circulation passages are arranged such that each first main passage 46A for bearing a clockwise torque and the adjacent second main passage 46B for bearing a counterclockwise torque make a pair. Namely, shoulder portions 48 each interposed between the corresponding pair of first and second main passages 46A and 46B are formed on the outer periphery of the screw shaft 42. Further, shoulder portions 49 each opposed to the corresponding first main passage 46A from the counterclockwise direction are formed on the inner periphery of the spline-purpose outer tube 44, and also shoulder portions 50 each opposed to the corresponding second main passage 46B from the clockwise direction are formed on the inner periphery of the outer tube 44. With this construction, the spline-purpose balls 45 disposed in the first main passages 46A bear a clockwise torque, and the spline-purpose balls 45 disposed in the second main passages 46B bear a counterclockwise torque. Further, shoulder portions 51 each interposed between the corresponding pair of first and second main passages 46A and 46B are formed on the inner periphery of the spline-purpose outer tube 44, and each shoulder portion 51 is opposed to a corresponding spline groove 52 (formed in the inner periphery of the spline-purpose outer tube 44) such that a clearance is formed between the shoulder portion 51 and the spline-purpose balls 45.
As a result, when a force acting in the clockwise direction is applied to the nut 44, the balls 45 indicated by A in FIG. 5 bear this torque, and these balls will not bear a torque when a force acting in the counterclockwise direction is applied to the nut 44. When a force acting in the counterclockwise direction is applied to the nut 44, the balls 45 indicated by B in FIG. 5 bear this torque, and these balls will not bear a torque when a force acting in the clockwise direction is applied to the nut 44. The balls 45 disposed in the return passages 47 do not bear the clockwise torque and the counterclockwise torque.
In the ball spline-equipped ball screw employing the torque bearing structure shown in FIG. 5, there has been encountered a problem that a torque capacity is small, and therefore it has been required to increase the torque capacity without increasing the diameter of the shaft.